This invention relates to radiation curable 100% solids coatings and adhesives compositions containing cyanoalkoxyalkyl and cyanothiaalkylalkyl acrylates as a non-volatile, non-toxic and highly polar reactive monomer (reactive diluent).
It is well known in the art to deposit polymerizable liquid coatings on substrates and then to dry and cure such coatings by passing them through ovens to crosslink the coatings. It is also known in the art to use coatings which are curable by exposure to actinic radiation such as ultraviolet rays or that from plasma arc radiation sources, as well as by exposure to high energy ionizing radiation such as electron beam radiation. These systems have disadvantages in that either ovens are required or that, in radiation curable coatings, oxygen inhibition of the polymerization of the coating may occur, or it may be necessary to employ solvents to decrease the viscosity of the coating for application purposes.
Due to energy and raw material shortages, increasingly strict air pollution standards and safety regulations, the search is continuing for 100% polymerizable systems, i.e., compositions which have no highly volatile components but which contain reactive viscosity reducing diluents which become either the sole cured film or part of the cured film. Such types of compositions are known, for example, multifunctional acrylates, methacrylates and itaconates of pentaerythritol, dipentaerythritol and polypentaerythritols and others disclosed in U.S. Pat. Nos. 3,551,235; 3,551,246; 3,551,311; 3,552,986; 3,558,387; and 3,661,614.
Two U.S. patents which disclose radiation cure of monofunctional acrylates are U.S. Pat. No. 3,783,006 which describes a wide number of acrylate monomers for use as diluents in polymer syrups which are cured on metal containers via electron beam radiation and U.S. Pat. No. 3,772,062 which also describes the use of various diluents in the curable coatings.
U.S. Pat. No. 4,097,677 discloses dicyclopentenyloxyalkyl (meth)acrylates and compositions thereof that can be cured by exposure to radiation.
U.S. Pat. No. 4,148,987 discloses monoethylenically unsaturated derivatives of vinylbenzyl-substituted benzophenones or acetophenones, and homopolymers and copolymers thereof that can be cured by exposure to radiation.
U.S. Pat. No. 3,650,669 discloses a method for the polymerization of a variety of monomers and for the crosslinking of polymers by exposing the monomers or the polymers to radiation. Column 6, lines 34-50 disclose the curing or crosslinking of 100% solids coating compositions containing a polymer with a reactive monomer, particularly polyesters containing the norbornenyl group with certain high boiling acrylyl esters, among which esters there is mentioned at column 11, line 56 "cyanoalkyl" esters of (meth)acrylic acid wherein the alkyl group contains up to 15 carbon atoms.
Monomers which are suitable for cure by exposure to radiation (i.e. ultraviolet light or electron beam radiation) require a wide range of properties, such as low-volatility, rapid polymerization rate, low toxicity, and low odor. Additionally, radiation curable compositions frequently contain dissolved polymers or oligomers so that the ability of the monomers to solubilize polymers with substantial reduction in viscosity becomes important. Since the monomer portion of all these compositions contacts the substrates its properties influence adhesion, flow, wetting, and so on. In textile applications it is necessary that the monomer be polar enough to yield a polymer with substantial resistance to swelling in drycleaning solvents.
The dicyclopentenyloxyalkyl (meth)acrylates yield polymers characterized by high glass transition temperatures and are well suited for paints and similar coatings on rigid substrates.
Other low volatility acrylate monomers, such as the polyol polyacrylates, are available; however, these cure to highly crosslinked and extremely hard, brittle films. Higher alkyl acrylates such as lauryl acrylate have the desired lower volatility and viscosity but their use alone as a diluent tends to promote excessive softening. Use of such higher alkyl acrylates with the dicyclopentenyloxyalkyl (meth)acrylates provides a satisfactory degree of latitude of hardness and flexibility and viscosity control not heretofore obtainable.
There is still a need for radiation curable monomers which give low glass transition temperature (Tg), flexible, and polar polymers. Butyl acrylate and 2-ethylhexyl acrylate are too volatile for general use. The higher alkyl acrylates give slower cures, are relatively non-polar, and are poor solvents for the oligomers and polymers used. The hydroxyl functional acrylates, e.g. hydroxyethyl acrylate, although having good solvency and resistance properties, are toxic and cause serious skin irritation. Similarly, ethoxyethyl acrylate is a skin irritant.
The .beta.-cyanoethoxyalky acrylates of the patent application of Emmons, Winey and Rounds filed simultaneously herewith provide a class of radiation curable reactive monomers possessing a desirable balance of high polarity, low volatility, and rapid polymerization rate. However, these monomers are made using acrylonitrile, a material reported to possess toxic and mutagenic properties. In the trans-esterification reaction of the intermediate cyanoethoxyalkanol with the acrylic acid ester, the intermediate can decompose from a non-toxic material to the toxic starting material, acrylonitrile, increasing the hazards associated with the use of acrylonitrile.